Contents
How is latch inferred prevented?
How to mitigate the risk of unintended latches:
- Make intended latches simple and identifiable:
- All combinatorial logic blocks need to be defined with always @* or SystemVerilog’s always_comb .
- Make sure all variables assigned in a combinatorial logic blocks have an initial or default assignment.
What is a latch in Verilog?
A flip-flop captures data at its input at the negative or positive edge of a clock. A latch does not capture at the edge of a clock; instead, the output follows input as long as it is asserted. The D latch is used to store one bit of data.
What is inferred latch and how it is created?
In an always construct at the specified location in a Verilog Design File (. v), you updated the value of the specified variable. When a variable asynchronously holds its previous value under certain conditions, the variable will infer a latch.
Where can a latch be generated in Verilog?
Another place where a latch could be generated in Verilog is in a case statement that does not include the default assignment. If not all possible cases are accounted for and the case is in a combinational always block then the synthesis tools will infer a latch.
How to avoid latches in combinational always blocks?
To avoid this, make sure that all of your if statements that are used in combinational always blocks always have one last else statement to catch all missing conditions! Another place where a latch could be generated in Verilog is in a case statement that does not include the default assignment.
How to avoid latches in your FPGA code?
Let’s look at one more example in Verilog: This code above will generate a latch, because the output o_latch is not defined when the input i_select is equal to 2’b11. To avoid this, make sure that all of your if statements that are used in combinational always blocks always have one last else statement to catch all missing conditions!
How are latches created in a VHDL process?
Latches are created when you create a combinational process or conditional assignment (in VHDL) or a combinational always block (in Verilog) with an output that is not assigned under all possible input conditions. This creates what is known as incomplete assignment by the synthesis tools.